I've seen a thread where the brother sawed the top 2" or so off the top edge of the kettle "bowl", then inserted it into to top of the drum so the dome lid would seat properly. They had some gasket rope fixed to the rim of the insert to prvide a better seal. That might be an option you'd consider.
Here's a thread with one example: http://www.bbq-brethren.com/forum/sh...ad.php?t=40875
1 x UDS, 1 x 22.5" Weber kettle, 1 x COS(for storage), 1 x Kingsford SJS knockoff, 1 x ailing 2 burner gasser.